Thirty years of stream protection Long-term nutrient and vegetation changes in a retired pasture stream SCIENCE FOR CONSERVATION 300 Thirty years of stream protection Long-term nutrient and vegetation changes in a retired pasture stream Clive Howard-Williams and Stuart Pickmere SCIENCE FOR CONSERVATION 300 Published by Publishing Team Department of Conservation PO Box 10420, The Terrace Wellington 6143, New Zealand Cover: Whangamata Stream upper section looking downstream from Whangamata Road Bridge in 2008—32 years after retirement from pasture. Photo: Authors’ collection. Science for Conservation is a scientific monograph series presenting research funded by New Zealand Department of Conservation (DOC). Manuscripts are internally and externally peer-reviewed; resulting publications are considered part of the formal international scientific literature. Individual copies are printed, and are also available from the departmental website in pdf form. Titles are listed in our catalogue on the website, refer www.doc.govt.nz under Publications, then Science & technical. © Copyright March 2010, New Zealand Department of Conservation ISSN 1173–2946 (hardcopy) ISSN 1177–9241 (web PDF) ISBN 978–0–478–14743–8 (hardcopy) ISBN 978–0–478–14744–5 (web PDF) This report was prepared for publication by the Publishing Team; editing by Amanda Todd and layout by Hannah Soult. Publication was approved by the General Manager, Research and Development Group, Department of Conservation, Wellington, New Zealand. In the interest of forest conservation, we support paperless electronic publishing. When printing, recycled paper is used wherever possible. CONTENTS Abstract 5 1. Introduction 6 2. Aims 8 3. Methods 9 3.1 Site description 9 3.2 Sampling 9 4. Results 10 4.1 Vegetation 10 4.1.1 Upper reach (Sections A and B) 14 4.1.2 Middle reaches (Sections C, D and E) 14 4.1.3 Lower reaches (Sections F and G) 14 4.2 Flow rates 15 4.3 Total suspended solids (TSS) 16 4.4 Nutrients in the Right Hand Spring and Left Hand Tributary 17 4.5 Nutrients in the stream channel 18 4.5.1 Nitrate-N 18 4.5.2 Dissolved reactive phosphorus (DRP) 20 4.5.3 Ammonium-N 21 4.5.4 Dissolved organic nutrients 22 4.5.5 Particulate and total nitrogen and phosphorus 22 4.6 Nutrient removal 22 5. Discussion 24 5.1 Vegetation trends 24 5.2 Stream flow trends 26 5.3 Catchment modifications 26 5.4 Nutrient attenuation processes 27 6. Recommendations 31 6.1 Vegetation management 31 6.2 Management for nutrient uptake 32 6.3 Flow monitoring 33 6.4 Water quality monitoring 33 6.5 Publicity 33 7. Acknowledgements 34 8. References 34 Appendix 1 Nutrient concentrations in Whangamata Stream, 1995–2008 36 Appendix 2 Vascular plants of the Whangamata Stream, March 2008 45 Thirty years of stream protection Long-term nutrient and vegetation changes in a retired pasture stream Clive Howard-Williams and Stuart Pickmere National Institute of Water & Atmospheric Research Ltd, 10 Kyle Street, PO Box 8602, Riccarton, Christchurch 8440, New Zealand Email: [email protected] Abstract Whangamata Stream, which flows into Lake Taupo (Taupomoana), New Zealand, was protected by the establishment of riparian strips in 1976. The stream has been monitored since 1978, and the findings from this monitoring programme are analysed in this report. The vegetation in the riparian zone developed from a ryegrass-clover (Lolium-Lotus) pasture to a mixture of exotic woodland and native scrub communities, with dense flax (Phormium tenax), sedges (Carex spp.) and toetoe (Cortaderia fulvida) overhanging the stream channel. With the help of conservation plantings, the number of species increased at an average rate of 5.2 species per year (6.6% per year). There has been a 7.2% annual turnover in species and a continuing increase in the proportion of woody species over time. Over the 32-year period, base-flow stream discharge has varied from 0.03 m3/s to c. 0.166 m3/s, with some long-term cycles apparent. In the source springs, nitrate concentrations have increased by 50% since 1984, while dissolved reactive phosphorus concentrations have only increased slightly. In the stream channel, there was marked seasonality in both the concentration and mass flow of total suspended solids and nutrients, with lowest values in summer at the time of maximum vegetation growth and therefore nutrient uptake. Active management of stream bank vegetation that minimises shade over the stream channel and maximises fast-growing stream plants is required if the management objective for the riparian strip is to enhance in-stream nutrient uptake (attenuation). If, however, the long-term objective is to restore the stream banks to a ‘natural state’—in this case a manuka (Leptospermum scoparium) woodland—then dissolved nutrient attenuation will decrease with time. Riparian protection and managed enhancement of in-stream vegetation for nutrient removal needs to be considered as one of a range of nutrient control methods in farming catchments. Keywords: riparian strips, restoration, nitrogen, phosphorus, wetland, stream rehabilitation, succession, biodiversity, nutrient attenuation © Copyright March 2010, Department of Conservation. This paper may be cited as: Howard-Williams, C.; Pickmere, S. 2010: Thirty years of stream protection: long-term nutrient and vegetation changes in a retired pasture stream. Science for Conservation 300. Department of Conservation, Wellington. 49 p. Science for Conservation 300 5 1. Introduction Changing land uses in rural New Zealand have been associated with changing water quality (MfE 2007). In the face of intensified land use, riparian management has been the most commonly used mitigation measure for protecting water quality, and stream restoration efforts in New Zealand’s agricultural landscapes have largely focused on management of riparian areas that typically involves excluding livestock and planting with native trees and shrubs (MfE 2001). However, there are few long-term studies of streams that document in detail the time scales over which changes occur following such protection. This report records the findings from the long-term study of the Whangamata Stream, a second-order spring-fed stream that flows into the northern side of Lake Taupo (Taupomoana). This stream was retired from pastoral agriculture in 1976 as part of the Lake Taupo Catchment Control Scheme, by the establishment of riparian strips along the stream margins (Fig. 1). Although the primary aim of the stream protection was originally to minimise the impacts of soil erosion (Waikato Valley Authority 1973), reduction of the effects of nutrient runoff from farmland was also seen as an important objective. Since then, the emphasis on nutrients has grown, owing to concern over the increasing nutrient loads entering Lake Taupo (Taupomoana) (MfE 2002). Since 1976, land use in the Whangamata Stream catchment has changed markedly from purely pastoral agriculture to forestry and life-style block development, and more recently has included urban development. Discussions with the former owner of the land (Mr R. Holyoake, pers. comm.) indicated that before conversion to sheep pasture, the stream edges were originally occupied by manuka (Leptospermum scoparium). This matched the description of the vegetation of much of the area surrounding Lake Taupo (Taupomoana) as manuka and fern scrubland (Ward 1956). Ward’s ‘Taupo Country’ land use map of 1954 shows the Whangamata Stream catchment as a mix of pasture, newly-turned land and burnt-over land (Ward 1956). After conversion to pastoral farming, the banks were grazed as ryegrass-clover (Lolium- Lotus) pasture from the 1950s to 1976. Following the riparian fencing of most of the stream banks in 1976, the process of rehabilitation of the stream margins was assisted by some plantings of native species, including the stream-bank wetland species flax (Phormium tenax) and toetoe (Cortaderia fulvida) on the pasture-grassed banks. These plantings were initiated in a few areas by the then Waikato Valley Authority (now Environment Waikato), the New Zealand Wildlife Service and in later years by its successor, the Department of Conservation (Howard-Williams & Pickmere 1999). Over the 32 years of protection (1976–2008), there have been a significant number of research papers and popular articles on this stream, documenting changes throughout the period. Since 1994, an annual monitoring report has been produced for the Department of Conservation and Environment Waikato. In 1999 and 2005, the Department of Conservation produced two publications in their Science for Conservation series that compiled and reported on monitoring reports for the previous 5 and 7 years (Beadel 1993, 1998; Howard-Williams & Pickmere 1999, 2005; Wildland Consultants Ltd 2003). These described 6 Howard-Williams & Pickmere—Thirty years of stream protection Figure 1. The study section of the Whangamata Stream showing the riparian strip (thin white outline), the sampling sites and the seven sections (A–G) for the vegetation descriptions outlined in section 4.1. Stream channel (thick white line) location is approximate. Distance between Top Site and Bottom Site is c. 2 km. Modified from original diagram in Wildland Consultants 2008 (with permission). vegetation changes, flow rates and water quality in the stream at an upper (‘Top Site’) and lower (‘Bottom Site’) sampling point, and in the spring waters feeding the stream. Steps in the stream restoration process between 1976 and 1993 were identified (Young 1980; Howard-Williams & Pickmere 1994) and were recorded in the book ‘Restoration of aquatic habitats’ (Collier 1994); these were then added to in 1999 (Howard-Williams & Pickmere 1999). Over time, there have been significant improvements to wildlife, rainbow trout (Oncorhynchus mykiss) spawning and biodiversity values in the stream (Young 1980; Howard-Williams & Pickmere 1999, 2005), and a series of changes in water quality parameters associated with changing riparian vegetation and changing discharges from the two springs feeding the stream. Recent interest in the long-term protection of Lake Taupo (Taupomoana) (MfE 2002) has focused on nutrients entering and then travelling in the groundwater towards the lake.